I always sprayed and applied granular at the same time, with my Ultra. You are correct that on small lawns, it is quicker to treat with the hose (providing that you are applying fert and herbicide at the same time). Naturally lawns cut up into small areas need the hose. Also, use the hose on hills too steep to safely navigate. I normally use the hose on lawns of 8000 sqft or less and lawns of 9000 or 10,000 were a toss-up...could go either way.
I had a special spot on my truck behind the cab with a bungee cord bolted on and dedicated to empty bags.